This invention relates to a multi-fuel engine control for a vehicle and particularly to such a control having induction apparatus for providing air and a liquid fuel mixture of two combustible fuels having different volatilities to the engine combustion chambers in a predetermined air/fuel ratio. Such a vehicle also includes a fuel tank and is often provided with fuel vapor canister apparatus communicating with the fuel tank to store vaporized fuel therefrom.
The canister is purged during engine operation by canister purge apparatus which throttles a conduit between the canister and engine induction apparatus to control the induction of fuel vapors. The primary concern in the induction of such vapors into the engine is that, since the air/fuel ratio of the contents of the canister and vapors coming from the tank is uncontrolled, the induction rate should be limited to prevent the predetermined air/fuel ratio from being significantly changed or, if a closed loop control is included, to prevent this control from being overwhelmed. A typical canister control for a gasoline fueled engine varies the throttle opening between the canister and engine with engine speed, so that more canister vapors may be drawn into the engine at high speeds to match the greater fuel and air flow to the engine in a controlled air/fuel ratio.
The presence in the fuel mixture of a second fuel such as methanol, which has a different volatility from that of gasoline, complicates the situation, since the volatility of the fuel mixture can vary greatly with the relative proportions of gasoline and methanol in the fuel as well as with fuel temperature. With respect to fuel composition, for example, the vaporization rate of methanol is significantly less than that of gasoline at a typical given temperature; but the vaporization rate of a mixture of the two, at least in some proportions, may be significantly higher than that of either alone.